The present invention relates to a light deflector utilizing holograms.
Conventional light deflectors include mechanical light deflectors such as rotary multimirrors and galvanometers, ultrasonic light deflectors which utilize the interaction between an ultrasonic wave and a light beam, and hologram light deflectors which utilize moving holograms. In general, hologram light deflectors are less costly than the other types of light deflectors because the hologram (which is a major component thereof) can be reproduced in large quantities by either a photographic process or a thermal pressing process. In addition, the hologram itself has an inherent lens effect, obviating the need to use a condenser lens.
In a hologram light deflector, the direction of a first-order reconstructed diffraction light is varied by changing the relative positional relationship between a hologram plate having a lens effect and a reconstructing illumination light. Such hologram light deflectors include an array of holograms recorded along the circumference of a rotating disk, an array of reflection holograms recorded on a rotating spherical or concave surface along its direction of rotation, or an array of holograms recorded along a side surface of a rotating cylinder or prism.
The hologram light deflector comprises a plurality of holograms which reconstruct a plurality of scanning beams during the course of one revolution of the deflector. Typically, the holograms are spaced at regular intervals and are scanned by scanning beams clocked at correspondingly regular intervals, producing even scanning lines. However, if the holograms are spaced at irregular intervals, the scanning of the holograms with scanning beams clocked at regular intervals produces uneven scanning lines because the scanning starting points vary from hologram to hologram. As a result, jitter is created which distorts the image formed by the scanning lines. Previous attempts to reduce this problem by increasing the accuracy of the fabrication of the hologram light deflector have been both costly and subjected to technical limitations.
A method of aligning scanning starting points by detecting variations in the intensity of zero-order light at the time of hologram reconstruction has been described in Japanese Patent Laid-Open Publication No. 55-38530. By using this method, the scanning starting points can be aligned by a simple arrangement without the need for improving the fabrication accuracy of the hologram light deflector. However, this method is insufficient for precisely positioning the scanning beam throughout its full scanning range. Specifically, when the hologram light deflector rotates irregularly, the accuracy of the position of a scanning point which is generated by signals cannot be compensated. This is due to the fact that whether picture elements are generated (by clocks at constant intervals after the scanning has started) or signals for reading out picture images are generated, the position of the scanning beam is ultimately dependent upon either the accuracy of a driving device or the accuracy with which the holograms are attached to the deflector.